Recently, various flat panel display devices have been developed which can eliminate disadvantages of cathode ray tubes (CRTs) caused by bulky and heavy structures thereof. Such flat panel display devices include a liquid crystal display (LCD), a field emission display (FED), a plasma display panel (PDP), and a light emitting display (LED).
Recently, the importance of display devices as visual information media has been emphasized, so various flat panel display devices have been developed.
Such flat panel display devices include a liquid crystal display (LCD), a field emission display (FED), a plasma display panel (PDP), an electroluminescence (EL), etc.
Of such flat panel display devices, the LCD exhibits an expanded application range by virtue of lightness, thinness, low power consumption, etc. In accordance with the expanded application range, the LCD is used for a portable computer such as a notebook PC, an office automation appliance, an audio/video appliance, an indoor/outdoor advertising display, etc. The LCD has also been rapidly developed toward a large size and a high resolution, by virtue of the development of techniques for mass production and the results of research and development.
A general LCD device includes an LCD module, and a driving circuit for driving the LCD module.
In detail, the LCD device includes a liquid crystal panel having liquid crystal cells arranged in the form of a matrix between two glass substrates, a back light unit for irradiating light to the liquid crystal panel, and a driving circuit for driving the liquid crystal panel and back light unit.
In the LCD module, a plurality of optical sheets are arranged to change the travel direction of light traveling from the back light unit toward the liquid crystal panel such that the light is directed in a direction perpendicular to the liquid crystal panel. The liquid crystal panel, back light unit, and optical sheets should be coupled together to form an integrated structure, in order to avoid optical loss. They should also be protected from damage caused by an external impact. To this end, a case for the LCD device, which is formed to enclose the periphery of the liquid crystal panel, back light unit, and optical sheets, is provided.
FIG. 1 is a cross-sectional view illustrating a general LCD module using a direct type back light unit.
Referring to FIG. 1, the back light unit 10 of the LCD device includes a plurality of lamps 30 for irradiating light to a liquid crystal panel (not shown) as a display screen, a diffuser plate 40 for diffusing the light incident from the lamps 30, thereby causing the diffused light to be irradiated to the liquid crystal panel, a lamp housing 20 arranged at the backside of the lamps 30, and a plurality of optical sheets 50 laminated on the diffuser plate 40.
For each lamp 30, a cold cathode fluorescent lamp is mainly used. Each lamp 30 includes a glass tube, an inert gas contained in the glass tube, and a cathode and an anode respectively mounted to opposite ends of the glass tube. Light emitted from each lamp 30 is incident to the diffuser plate 40.
The number of required lamps 30 increases in proportion to the size of the display screen. In the case of an LCD device having a screen size of 40 to 42 inches, 14 to 16 lamps are mounted. Also, the lamps 30 are arranged such that the adjacent lamps 30 are spaced apart from each other by a distance “W1 (22 mm)”.
In the lamp housing 20, a reflection sheet or a reflection plate is mounted to reflect light toward the diffuser plate 40. The lamps 30 are also mounted in the lamp housing 20. The reflection sheet or reflection plate is made of a material capable of reflecting light. The reflection sheet or reflection plate is formed to correspond to the bottom surface and inclined side surfaces of the lamp housing 20. Accordingly, the reflection sheet or reflection plate reflects light toward the diffuser plate 40, thereby enhancing the efficiency of light irradiated to the liquid crystal panel. Here, the lamp housing means a bottom chassis of the LCD device.
The diffuser plate 40 causes incident light from the lamps 30 to be directed to the front of the display surface while diffusing the light such that the light is uniformly distributed over a wide range. The diffuser plate 40 irradiates the diffused light to the liquid crystal panel.
The light emitted from each lamp 30 is incident to the display screen via the diffuser plate 40 and optical sheets 50. The light emerging from the diffuser plate 40 is diffused light having a large viewing angle. The light incident to the display screen exhibits an increased light efficiency when it is perpendicular to the display screen. In order to enhance the efficiency of the light emerging from the diffuser plate 40, a plurality of optical sheets 50 are arranged on the diffuser plate 40.
FIG. 2 is an enlarged sectional view of a part of the back light unit shown in FIG. 1.
Referring to FIG. 2, the plurality of optical sheets 50, for example, first to third optical sheets, change the direction of the light emerging from the diffuser plate 40 such that the light is directed in a direction perpendicular to a display screen, to achieve an enhancement in light efficiency. To this end, the back light unit includes a first diffuser sheet 52 for diffusing light emerging from the diffuser plate 40 over the overall region of the display screen, a prism sheet 54 for changing the traveling angle of the light diffused by the first diffuser sheet 52 such that the diffused light is directed in a direction perpendicular to the display screen, and a second diffuser sheet 56 for diffusing the light emerging from the prism sheet 54 over the overall region of the liquid crystal panel while enhancing the efficiency of the light.
The second optical sheet may be substituted by a diffuser sheet. If necessary, the third optical sheet may be substituted by a dual brightness enhancement film (DBEF), in order to achieve a further increase in brightness. Accordingly, the light emerging from the diffuser plate 40 is incident to the display screen via the plurality of optical sheets 50.
However, the conventional back light unit 10, which has a general configuration as described above, requires a certain distance from the plurality of lamps 30 to the diffuser plate 40, in order to obtain a uniform brightness at the display screen.
As shown in FIG. 1, the conventional back light unit 10 has a large thickness “d1 (37 mm)” because the distance from the plurality of lamps 30 to the diffuser plate 40 is long. Since the back light unit 10 has a large thickness of 37 mm, there is a drawback in terms of an important factor of the flat panel display device, namely, thinness.
In addition, although the conventional back light unit 10 uses a plurality of optical sheets 50 to achieve an enhancement in the uniformity of light irradiated to the display screen, a brightness difference corresponding to a value “A” is generated on the display screen because the brightness at a portion of the display screen arranged directly over each lamp 30 is high, whereas the brightness at a portion of the display screen arranged between the adjacent lamps 30 is low, as shown in FIG. 3. Such a brightness difference causes a degradation in the display quality of the display device.
In order to reduce the brightness difference, it is necessary to increase the distance from the plurality of lamps 30 to the diffuser plate 40. In this case, however, the LCD device has a further increased thickness. Meanwhile, an auxiliary light source may be mounted between the adjacent lamps, to achieve an enhancement in brightness uniformity. In this case, however, an increase in manufacturing costs occurs because the total number of light sources increases due to the provision of the auxiliary light source.